This invention relates to an exhaust system for outboard motors and more particularly to an improved exhaust system that offers better silencing both under high speed and low speed running.
As is well known, it has been the practice to discharge the exhaust gases of the engine of an outboard motor at a level beneath the water level during running at speed. The exhaust gases may be discharged through a hollow hub of the driving propeller or in proximity to the propeller so that some silencing effect will be accomplished under water. However, when the engine is operating at low speeds and the associated watercraft is traveling at a low speed, the underwater exhaust gas discharge will be submerged to a greater extent than when running at high speeds. Therefore, under slow speed running, the exhaust gases will be subjected to considerable back pressure and poor running and poor efficiency will result. To overcome this problem, it has been proposed to provide an above the water level slow speed exhaust for outboard motors. The exhaust gases are discharged through this slow speed exhaust under low speed running when the main exhaust outlet is submerged to a fairly large extent.
When an above water exhaust gas outlet is incorporated, it is obvious that there will be a substantial diminution of exhaust gas silencing. Although various devices have been proposed for silencing the above the water slow speed exhaust gas outlet, such devices have not provided the requisite degree of silencing. In addition, it is desirable to prevent the discharge of any significant amount of exhaust gases from the slow speed exhaust opening when operating at high speeds to prevent the discharge of unmuffled exhaust gases.
It is, therefore, a principal object of this invention to provide an improved slow exhaust gas silencing system for an outboard motor.
It is another object of this invention to provide an exhaust system for outboard motor having an above the water slow speed exhaust gas outlet that effectively silences the slow speed exhaust gases.
In addition to the problems of providing effective silencing for the slow speed exhaust gases, general silencing of exhaust gases in outboard motors presents several problems. Normally, the exhaust system of an outboard motor includes an exhaust pipe that extends from the exhaust outlet of the engine and which terminates in an expansion chamber. Such arrangements are normally tuned to effect silencing and both the volume of the expansion chamber and the length of the exhaust pipe are critical in achieving the desired silencing. In addition, it has been proposed to use resonant chambers either in lieu of or in combination with expansion chambers to further improve exhaust gas silencing. However, the relatively small size of the outboard motor and its relatively short height has made it difficult if not impossible with prior art constructions to achieve the desired degree of silencing. Normally, the exhaust gas silencing chambers and exhaust pipe extend through the drive shaft housing of the motor. Of course, such drive shaft housings have only limited size and must also accommodate other components of the motor such as the drive shaft.
It is, therefore, a further object of this invention to provide an improved high efficiency exhaust silencing system for an outboard motor.
It is another object of this invention to provide an outboard motor exhaust silencing arrangement wherein silencing may be achieved over a wide range of engine speeds.